I also think that this is a good idea, but this is mostly a feature for international forums with several languages.

In this context a discussion I had with Ilia comes to my mind. FUDforum sends many notifications in the language of the user triggering it (for example by posting a new message). For international forums it would probably better if the notifications would be send in the language of the recipient.
I understand that this could have an negative impact on performance and involves major changes in the code. However, it seems Frank has plans to refactor the localizations (conversion to gettext). Maybe this problem can be reconsidered in this process?

Either you can redirect with htaccess or directly with an apache module, so I guess one can set up a forum copy on a different folder, but connect to the same database, since I think the default/allowed themes are controlled from the GLOBALS file and not the database, right?

First you must go to the Administration - then choice Plugin Manager
Then appears at bottom all plugin what will be available .
Choice there plugin Theme Router . And the end choice Save Configuration
Then when anybody what go to your forum Fudforum will be see
page in own langugae , what have set as the first language in a browser